1. Field of the Invention
The present invention relates to a lubricating oil composition and more particularly to a lubricating oil composition which is excellent in frictional characteristics, is decreased in changes with time of the frictional characteristics and further is excellent in stability against oxidation or oxidation stability, and thus which is useful as a lubricating oil for use in various parts such as an automatic transmission, a continuously variable transmission, a brake of a tractor, a power steering and so forth.
2. Description of Related Art
A lubricating oil to be used in parts including a wet clutch or a wet brake of an automatic transmission, a continuously variable transmission, a tractor and so forth is required to have such properties that frictional characteristics, oxidation stability, corrosion resistance and rust resistance are good, and transmission torque is large. An especially important requirement is that the ratio of coefficient of static friction to coefficient of kinematic friction as a measure of frictional characteristics is small and further that the change with time of the above ratio is small.
A lubricating oil having a high coefficient of static friction and result in good transmission torque has heretofore been known. This lubricating oil, however, has disadvantages in that frictional characteristics are not sufficiently satisfactory and shift shock is undesirably big.
In recent years, with miniaturization of cars and with increased production of FF (front engine, front wheel driven) cars, a tendency toward miniaturization of an automatic transmission and so forth has been increasingly developed. This miniaturization of the automatic transmission makes a driver more sensitive to the shift shock. Thus, in order to reduce the shift shock and to make a car more comfortable to drive, it has become a technical subject to improve frictional characteristics, especially at an initial stage.
In order to improve frictional characteristics, a lubricating oil containing a friction modifier has been proposed. This friction modifier-containing lubricating oil, however, is not sufficiently improved in frictional characteristics and further has a problem in that the frictional characteristics are reduced by degradation of oil due to its long term use (change with time). Moreover there is a tendency that the corrosion preventing ability drops.
As described above there has not yet been obtained a lubricating oil which possesses frictional characteristics which are good and are decreased in the change with time, and further which has a high transmission torque.